This invention relates to optical fiber connectors, and, more particularly, to connectors which are capable of connecting an optical fiber to a source or to another optical fiber with very low loss and with little sensitivity to lateral misalignment.
Although the present invention finds utility in the coupling of light from a source to an optical fiber, the present discussion concerning connector alignment problems will be limited to fiber-to-fiber connectors. The butt connection between the ends of two optical fibers will result in an insertion loss that is caused by various fiber misalignment parameters, examples of which are: (a) lateral misalignment between the axes of the two fibers, (b) longitudinal separation between the endfaces of the two fibers, and (c) angular misalignment between the axes of the two fibers. Since the butted fiber arrangement, wherein the two fiber endfaces are adjacent one another, is particularly sensitive to lateral displacement, this type of connector is difficult to use in field applications.
Beam expanders employing lenses or tapered fibers have been employed in in-line connectors for single-mode fibers which are extremely sensitive to lateral misalignment due to the small core diameters thereof. Although such beam expanders exhibit a reduced sensitivity to lateral displacement, they are more sensitive to angular misalignment. The art of aligning two connector halves is sufficiently advanced that such increased sensitivity to angular misalignment can be tolerated. Expanded beam connectors are therefore receiving a considerable amount of attention. However, the cost of lens-type expanded beam connectors is so high that they have not achieved widespread use.
The basic principal of tapered expanded beam connectors of the downtaper type is described in the publication K. P. Jedrzejewski et al. "Tapered-Beam Expander for Single-Mode Optical-Fiber Gap Devices", Electronics Letters, 16th January 1986, vol. 22, No. 2, pp. 105-106. That publication teaches a connector of the type wherein a single-mode fiber having a core refractive index n.sub.1 and a cladding refractive index n.sub.2 is threaded through a capillary tube of glass having a refractive index n.sub.3 which is slightly lower than n.sub.2. The capillary tube is uniformly heated to collapse it about the fiber. The central region of the combined fiber and capillary tube is then tapered to a minimum neck diameter of 40 .mu.m, which is appropriate for fiber handling and cleaving. A taper ratio of 4:1 is said to be adequate for minimizing insertion loss. The field is initially guided by, and substantially confined to, the core of the single-mode fiber. As the energy propagates through the taper toward the small diameter end thereof, the field spreads out and is eventually no longer guided by the core but is effectively guided by the waveguide consisting of the cladding and the capillary tube. The Jedrzejewski et al. publication teaches that the taper should be adiabatic since such a taper will suffer negligible loss through mode coupling, and equations are set forth therein defining the condition for a taper to remain adiabatic. The requirement that the taper be adiabatic has been heretofore widely accepted because it has been thought that all of the power coupled to modes other than the fundamental mode will be lost, thereby resulting in an unacceptable connector loss. In an adiabatically tapered structure such as that disclosed by Jedrzejewski et al., wherein the total coupler length of both connector halves is 2 cm (about the minimum adiabatic length), a maximum beam expansion of approximately four times can be achieved. The required length for such adiabatic connectors increases roughly quadratically with increased beam expansion.
It has also been thought that tapered beam expanders of the up taper type should be adiabatically tapered. Such uptapered beam expanders are described in the publications: N. Amitay et al., "Optical Fiber Tapers - A Novel Approach to Self-Aligned Beam Expansion and Single-Mode Hardware", Journal of Lightwave Technology, vol. LT-5, No. 1, January 1987, pp. 70-76; D. Marcuse, "Mode Conversion in Optical Fibers with Monotonically Increasing Core Radius", Journal of Lightwave Technology, vol. LT-5, No. 1, January 1987, pp. 125-133; and H. M. Presby et al., "Optical Fiber Tapers at 1.3 .mu.m for Self-Aligned Beam Expansion and Single-Mode Hardware", Journal of Lightwave Technology, vol. LT-5, No. 8, August 1987, pp. 1123-1128. The Amitay et al. and the Marcuse publications state that conversion of the fundamental mode to higher-order modes or radiation by the taper, which at the enlarged end can support multimode propagation, must be negligible if a very low excess coupling loss is to be maintained. The Presby et al. publication states that losses exceeding 1 dB are incurred for tapers having lengths up to 1 cm and that for longer lengths, i.e., more gradual tapers, the loss decreases. Presby et al. also state that a relatively gradual and smooth transition from fiber to taper takes place over a length of about 6 cm and that no significant amount of mode conversion takes place in the taper. Such adiabatic taper lengths would result in inordinately long connectors.
The efficient coupling of light from a source to an optical fiber is also an important requirement in optical transmission systems. The design of a local area network or subscriber loop is critically dependent on the available optical power. As light propagates through the system, loss occurs, and eventually the optical power level becomes too low to be reliably detected. By increasing the efficiency of coupling light from a source such as a laser diode or LED into a single-mode fiber, system performance would be significantly enhanced. Various advantages could result from such an improvement. For example, low cost LED's might be substituted for high cost laser diodes.
Various methods are currently used to couple light from a source into a fiber, including butt-coupling, spherical and aspheric lenses, gradient-index lenses, and adiabatically tapered fibers. These methods can alter the alignment requirements for the fiber, but they cannot offer significant improvements in coupling efficiency because of modal-volume conservation.